Method and apparatus for performing communication by ue in wireless communication system supporting circuit switched fallback service

ABSTRACT

Provided is a User Equipment (UE) in a wireless communication system supporting a Circuit Switched FallBack (CSFB) service. The UE includes a transceiver for transmitting and receiving a signal to/from a first or second communication system that uses a different frequency; a receiver for receiving a signal from the second communication system; and a controller for controlling an operation of receiving signaling information for the CSFB service from the first communication system through the transceiver, and simultaneously measuring channel status of the second communication system using the signal received from the receiver.

TECHNICAL FIELD

The present invention relates generally to technology for setting upvoice calls in a Long Term Evolution (LTE) system, and moreparticularly, to a communication method and apparatus for reducing thetime required for setting up voice calls when Circuit Switched FallBack(CSFB) is used.

BACKGROUND ART

So far, the second-generation (2G) system and the third-generation (3G)system have been mainly used, and in recent years, standardization ofLong Term Evolution (LTE), which is the fourth-generation (4G) system,is under way.

Since the LTE system basically supports only the packet services, theLTE standard has proposed a so-called Circuit Switched FallBack (CSFB)service to provide voice services to users of the LTE system.

FIG. 1 schematically illustrates a configuration of a wirelesscommunication system supporting a CSFB function.

Referring to FIG. 1, an LTE system 110 includes an Evolved Packet Core(EPC) 111 and an evolved Node B (eNB) 113. Although not illustrated, theEPC 111 includes a Mobility Management Entity (MME) responsible for acontrol plane, and a Serving GateWay (S-GW) and a PDN GateWay (P-GW)responsible for a user plane. A 2G/3G legacy system 130 includes aMobile Switching Center (MSC) 131 responsible for call switchingprocessing, and a Base Station (BS) 133.

The CSFB service refers to a service, in which if a voice call occurs toa User Equipment (UE) 150 which is capping on the LTE system 110, the UE150 is handed over to the existing 2G/3G legacy system 130 to connectthe voice call.

During the CSFB process, the UE 150 is handed over from the LTE system110 to the 2G/3G legacy system 130. At this point, in order to select anappropriate cell (for example, a cell with a good channel status) fromamong the undepicted 2G/3G cells around the UE 150 and switch the UE 150to the selected cell, the LTE system 110 may receive measurement reportson the 2G/3G cells from the UE 150.

However, compared to when setting up a voice call directly in theexisting 2G/3G legacy system, if it uses CSFB, the UE 150 additionallyneeds the time required for switching from the LTE system to the 2G/3Glegacy system and measuring the cell status, causing delays that degradethe Quality of Service (QoS) in the CSFB service.

The above information is presented as background information only toassist with an understanding of the present disclosure. No determinationhas been made, and no assertion is made, as to whether any of the abovemight be applicable as prior art with regard to the present invention.

DISCLOSURE OF INVENTION Technical Problem

An aspect of the present invention is to address at least theabove-mentioned problems and/or disadvantages and to provide at leastthe advantages described below. Accordingly, an aspect of the presentinvention is to provide a method and apparatus for performingcommunication by a UE to reduce the delays in a wireless communicationsystem supporting CSFB.

Another aspect of the present invention is to provide a method andapparatus for performing communication by a UE to stably receive apaging message in a wireless communication system supporting CSFB.

Solution to Problem

In accordance with an aspect of the present invention, there is provideda User Equipment (UE) in a wireless communication system supporting aCircuit Switched FallBack (CSFB) service. The UE includes a transceiverfor transmitting and receiving a signal to/from a first or secondcommunication system that uses a different frequency; a receiver forreceiving a signal from the second communication system; and acontroller for controlling an operation of receiving signalinginformation for the CSFB service from the first communication systemthrough the transceiver, and simultaneously measuring channel status ofthe second communication system using the signal received from thereceiver.

In accordance with another aspect of the present invention, there isprovided a method for performing communication by a User Equipment (UE)that uses a Circuit Switched FallBack (CSFB) service if a voice call ofa second communication system occurs in a first communication systemproviding a data service. The method includes, if the voice call occurs,starting measurements on channel status of the second communicationsystem using a receiver for receiving a signal from the secondcommunication system; and sending and receiving messages for the CSFBservice to/from an evolved Node B (eNB) of the first communicationsystem using a transceiver for communication with the firstcommunication system, while performing the measurements on channelstatus.

Other aspects, advantages, and salient features of the invention willbecome apparent to those skilled in the art from the following detaileddescription, which, taken in conjunction with the annexed drawings,discloses exemplary embodiments of the invention.

BRIEF DESCRIPTION OF DRAWINGS

The above and other aspects, features and advantages of certainexemplary embodiments of the present invention will be more apparentfrom the following description taken in conjunction with theaccompanying drawings, in which:

FIG. 1 schematically illustrates a configuration of a wirelesscommunication system supporting a CSFB function;

FIG. 2 is a block diagram illustrating a structure of a UE according toan exemplary embodiment of the present invention;

FIG. 3 is a flowchart illustrating a method for performing communicationby a UE that uses a CSFB service, according to an exemplary embodimentof the present invention;

FIG. 4 is a flow diagram illustrating a communication method in awireless communication system supporting CSFB according to an exemplaryembodiment of the present invention;

FIG. 5 is a block diagram illustrating a structure of a UE according toanother exemplary embodiment of the present invention;

FIG. 6 is a flowchart illustrating a method for performing communicationby a UE that uses a CSFB service, according to another exemplaryembodiment of the present invention;

FIG. 7 is a flowchart illustrating a UE's operation of performingmeasurements on a legacy network according to another exemplaryembodiment of the present invention;

FIG. 8 is a flow diagram illustrating a communication method in awireless communication system supporting CSFB according to anotherexemplary embodiment of the present invention;

FIG. 9 is a flow diagram illustrating a communication method in awireless communication system supporting CSFB according to furtheranother exemplary embodiment of the present invention;

FIG. 10 is a flow diagram illustrating a procedure for sending a NASmessage including Dual RX configuration information according to furtheranother exemplary embodiment of the present invention; and

FIG. 11 is a flow diagram illustrating a procedure for providinginformation about cells for a CSFB service as system information by aneNB according to an exemplary embodiment of the present invention.

Throughout the drawings, like reference numerals will be understood torefer to like parts, components, and structures.

MODE FOR THE INVENTION

The following description with reference to the accompanying drawings isprovided to assist in a comprehensive understanding of exemplaryembodiments of the invention as defined by the claims and theirequivalents. It includes various specific details to assist in thatunderstanding but these are to be regarded as merely exemplary.Accordingly, those of ordinary skilled in the art will recognize thatvarious changes and modifications of the embodiments described hereincan be made without departing from the scope and spirit of theinvention. In addition, descriptions of well-known functions andconstructions may be omitted for clarity and conciseness.

The terms and words used in the following description and claims are notlimited to the bibliographical meanings, but, are merely used by theinventor to enable a clear and consistent understanding of theinvention. Accordingly, it should be apparent to those skilled in theart that the following description of exemplary embodiments of thepresent invention is provided for illustration purpose only and not forthe purpose of limiting the invention as defined by the appended claimsand their equivalents.

It is to be understood that the singular forms “a,” “an,” and “the”include plural referents unless the context clearly dictates otherwise.Thus, for example, reference to “a component surface” includes referenceto one or more of such surfaces.

Although exemplary embodiments of the present invention will bedescribed in detail below mainly for the Third Generation PartnershipProject (3GPP) Long Term Evolution (LTE) system, it will be apparent tothose of ordinary skill in the art that the embodiments may be appliedto any other communication/computer systems having the similar technicalbackgrounds and system configurations with a slight modification,without departing from the spirit and scope of the invention.

The present invention is technology for reducing the time required forsetting up a voice call in a communication environment where an LTEsystem uses CSFB to provide a voice service to a UE.

More specifically, if a user of the LTE system presses a Call button ofa UE, or if a Mobile Terminating (MT) voice call occurs to the user ofthe LTE system, the UE switches to the 2G/3G legacy system through theCSFB process, and then sets up a voice call. In order to prevent thefailure to set up a voice call, the UE needs to select an appropriatecell (for example, a cell with a good channel status, i.e., a cell whosesignal strength is greater than or equal to a predetermined referencevalue or whose channel status satisfies predetermined conditions) fromamong the 2G/3G cells around the UE, and to be switched to the selected2G/3G cell.

To this end, the LTE system may instruct the UE to perform measurementreports on the 2G/3G cells. Generally, a UE may receive signalstransmitted from one system, at a time. Therefore, in order to measurethe channel status of the 2G/3G cells, the UE needs to switch from theLTE system to the 2G/3G cell of the 2G/3G legacy system during apredetermined measurement period. However, since performing signalingfor CSFB in the LTE system and measuring the channel status of the 2G/3Gcells may not be simultaneously carried out, delays may occur inperforming signaling for CSFB and measuring the channel status of the2G/3G cells, which may reduce the Quality of Service (QoS) of the CSFBservice and increase the power consumption of the UE.

Exemplary embodiments of the present invention propose ways to performmeasurements for selecting a 2G/3G cell while performing signalingrequired for the CSFB process in the LTE system.

FIG. 2 is a block diagram illustrating a structure of a UE according toan exemplary embodiment of the present invention.

Referring to FIG. 2, a UE includes an LTE/legacy transceiver(hereinafter referred to as a ‘transceiver’ for short) 203 fortransmitting and receiving signals to/from the LTE system or 2G/3Glegacy system (not shown) via an antenna 201 in accordance with itsassociated predetermined communication scheme, and a measuring receiver207 for receiving measurement signals from 2G/3G cells of the 2G/3Glegacy system via an antenna 205. The UE may also include a storage 209for storing information about the cell on which it has camped, or thecells, the channel status of which it will measure.

A controller 211 controls and manages the overall operation of (forexample, voice calls, data calls, and application operations) of the UE.In the CSFB service, the controller 211 controls UE's operation for theuse of the CSFB service by sending a related request to the system orhandling a command message received from the system.

As one of its basic control operations, the UE may receive informationabout the cell on which it has camped, or the cells, the channel statusof which it will measure, through the transceiver 203, and store thereceived information in the storage 209. If the CSFB process is needed,the controller 211 controls an operation of performing measurement onthe 2G/3G cells of the 2G/3G legacy system by means of the measuringreceiver 207 in parallel (or simultaneously) while performing signalingfor CSFB with the LTE system by means of the transceiver 203.

FIG. 3 is a flowchart illustrating a method for performing communicationby a UE that uses a CSFB service, according to an exemplary embodimentof the present invention.

Referring to FIG. 3, in step 301, the UE initiates an operation of usingthe CSFB service as its user initiates an outgoing call, in the case ofa Mobile Originating (MO) call, and the UE initiates an operation ofusing the CSFB service as it receives a paging or CS paging notificationmessage from the LTE system, in the case of Mobile Terminating (MT)call. For the known initiation process of the CSFB service, referencemay be made to 3GPP TS 23.272.

If CSFB is initiated in step 301, the UE starts measuring the channelstatus of its nearby 2G/3G cells using the measuring receiver 207 instep 303. The UE may create its Radio Resource Control (RRC) connectionto an evolved Node B (eNB) of the LTE system, and start the measurementson its nearby 2G/3G cells upon receiving a measurement command from theeNB. Since the measuring receiver 207 may operate independently of thetransceiver 203 under control of the controller 211, the UE maysimultaneously perform signaling (for example, RRC connection creation,extended service request transfer, Packet Switched Hand Over (PSHO)/cell change order/RRC connection release) to the LTE system for theCSFB process, while measuring the channel status of the 2G/3G cells. Instep 305, if its measurements on the nearby 2G/3G cells are completed,the UE reports the measurement results on the nearby 2G/3G cells, whichare obtained by the measuring receiver 207, to the eNB of the LTE systemusing the transceiver 203. Although not illustrated in FIG. 3, theoperation of reporting to the eNB by the UE may be performed togetherwith some of the CSFB process.

FIG. 4 is a flow diagram illustrating a communication method in awireless communication system supporting CSFB according to an exemplaryembodiment of the present invention.

Referring to FIG. 4, in step 401, a voice call occurs as a userinitiates an outgoing call, in the case of an MO call, and a voice calloccurs as a UE 410 receives a paging or CS paging notification messagefrom the LTE system, in the case of an MT call. As the voice calloccurs, a CSFB process is initiated. For the detailed CSFB initiationprocess, reference may be made to 3GPP TS 23.272.

As the CSFB process is initiated, upon detecting a need for CSFB, the UE410 measures 2G/3G cell status in a 2G/3G Radio Access Network (RAN) 430using the measuring receiver 207 in step 403. The 2G/3G cell statusmeasurements may be performed if the UE 410 detects the need for CSFB,or may be performed at stated intervals. The UE 410 may perform the2G/3G cell status measurements as soon as the CSFB process starts, likein the example of FIG. 4. Although not illustrated, if an eNB 420 sendsa measurement command to the UE 410 through RRC connection, the UE 410may start the 2G/3G cell status measurements by receiving themeasurement command.

If the 2G/3G cell status measurements are completed, the UE 410 reportsthe measurement results to the eNB 420 using the RRC connection. If theUE 410 is in an idle state, the UE 410 sets up an RRC connection to theeNB 420 in step 405, before reporting the measurement results.Therefore, step 405 is optional.

Thereafter, in step 407, the UE 410 sends an extended service request toan MME 440 that controls the UE mobility in the LTE system and isresponsible for the UE's control plane, in order to request the CSFBprocess. The extended service request is forwarded to the MME 440 viathe eNB 420.

The UE 410 may simultaneously perform the RRC and Non Access Stratum(NAS) signaling and the 2G/3G cell status measurements using thetransceiver 203. Upon receiving the extended service request, the MME440 signals (or sends) to the eNB 420 a command to fall back the UE 410to the 2G/3G legacy system, by using, for example, an S1 interface, instep 409. In step 411, upon receiving the command, the eNB 420 selects a2G/3G cell (or a list of cells) that the UE 410 will actually use, andsends, to the UE 410, a 2G/3G switching command (for example, PS HOcommand, cell change order, RRC connection release, or the like). Whenselecting a 2G/3G cell, the eNB 420 uses the cell status informationthat the UE 410 measured and reported in step 403. Thereafter, in step413, the known remaining procedure is performed to provide the CSFBservice to the UE 410.

Another exemplary embodiment of the present invention, which is proposedto prevent the failure to receive a paging message transmitted from theLTE system using CSFB, will be described in detail below.

Specifically, it will be assumed that when two or more communicationservices (for example, a communication service of the LTE system and acommunication service of the 2G/3G legacy system) are provided atdifferent frequencies, a UE frequently repeats cellselection/reselection (the so-called ping-pong phenomenon) as it islocated in the cell boundary, or due to the cell policy. In this case, aUE capable of using the two or more communication services may notstably receive a paging message transmitted from its associated system,using any one frequency.

For example, if a UE is considered that can use communication serviceswhich are provided from the LTE system supporting the CSFB service andthe 2G/3G legacy system using different frequencies, a paging message(for example, a paging message for informing the UE of the start of theCSFB) indicating that a voice call occurs from the 2G/3G legacy system,to provide the CSFB service in the LTE system, is sent to the UE throughthe LTE system, and is not sent to the UE through the 2G/3G legacysystem. In this case, if the UE has been camping on a cell of the 2G/3Glegacy system, the UE may not stably receive the paging message that issent to the UE through the LTE system, which causes an increase in callfailure rate in the wireless communication system.

Therefore, another exemplary embodiment of the present inventionprovides a communication apparatus and method for a UE, which includes atransceiver that can receive signals from the LTE system and/or the2G/3G legacy system to make it possible to stably receive the pagingmessage even in the above cases, and also includes a separate legacyreceiver capable of receiving signals from the 2G/3G legacy system.

FIG. 5 is a block diagram illustrating a structure of a UE according toanother exemplary embodiment of the present invention.

Referring to FIG. 5, a UE includes antennas 501 and 505, an LTE/legacytransceiver 503, a legacy receiver 507, a storage 509, and a controller511. The components of the UE in FIG. 5 may be the same as thecorresponding components of the UE in FIG. 2 in terms of the operation.Therefore, a description of the same components will be omitted, whenthey perform the same operation as the components of the UE in FIG. 2.

In accordance with the UE structure of FIG. 5, the UE receives signalsfrom the 2G/3G legacy system through the legacy receiver 507 even if theUE camps on a cell of the 2G/3G legacy system. Therefore, the UE maystably receive, through the LTE/legacy transceiver 503, a paging messagethat is sent to the UE through the LTE system for the CSFB service,thereby making it possible to increase the call success rate of the UEwhen the ping-pong phenomenon occurs.

The legacy receiver 507 may be implemented to include the function ofthe measuring receiver 207 in FIG. 2. In an alternative embodiment, thelegacy receiver 507 may be implemented to exclude the function of themeasuring receiver 207.

FIG. 6 is a flowchart illustrating a method for performing communicationby a UE that uses a CSFB service, according to another exemplaryembodiment of the present invention.

Referring to FIG. 6, the UE determines in step 601 whether the ping-pongphenomenon occurs in which cell selection/reselection is frequentlyrepeated in the wireless communication system supporting CSFB. Whetherthe ping-pong phenomenon occurs may be determined using the knowntechnology, so a detailed description thereof will be omitted.

If the ping-pong phenomenon occurs, the UE controls its receptionoperation of receiving signals from the LTE system through a firsttransceiver (for example, the LTE/legacy transceiver 503) and receivingsignals from the 2G/3G legacy system through a second receiver (forexample, the legacy receiver 507), in step 603. Thereafter, the UEdetermines in step 605 whether a CSFB-related paging message is sentfrom the LTE system. If so, the UE receives the CSFB-related pagingmessage through the first transceiver and performs the subsequentoperation for the CSFB service, in step 607.

Although the embodiment in FIG. 6 is an example of the operation of theUE with the structure of FIG. 5, it is to be noted that the operation ofthe UE in FIG. 5 is not limited to the communication method in FIG. 6.

Next, a description will be made as to when the UE will startmeasurements on the 2G/3G cells using the legacy receiver 507, when themeasurement method proposed by another exemplary embodiment of thepresent invention, i.e., the UE structure of FIG. 5 uses a method ofreducing the delay time occurring in the CSFB service with the use ofthe LTE/legacy transceiver 503 and the legacy receiver 507.

Generally, a user of a UE executes a dial or phonebook function to makea phone call. For example, if the UE is a smart phone, the time at whichmeasurements on the 2G/3G cells is started may be detected, as adial/phonebook application is executed. If the UE is a feature phone,the time at which measurements on the 2G/3G cells is started may bedetected, as a dial/phonebook button is pressed. While these operationsare executed, the user may consume time to enter a phone number or tosearch for a call recipient. During this time, the UE may performmeasurements on the legacy network.

FIG. 7 is a flowchart illustrating a UE's operation of performingmeasurements on a legacy network according to another exemplaryembodiment of the present invention. The operation in FIG. 7 will bedescribed with reference to FIG. 5.

Referring to FIG. 7, if the controller 511 determines in step 701 that auser makes an outgoing call by executing a dial or phonebook function,the controller 511 immediately starts measurements on the legacy networkusing the legacy receiver 507 in step 703. If a recipient's phone numberis entered/selected and then a Call button is pressed in step 705, thecontroller 511 performs random access to the LTE network as anintroduction process for the CSFB service using the LTE/legacytransceiver 503 in step 707. In an exemplary embodiment of the presentinvention, since the legacy receiver 507 and the LTE/legacy transceiver503 are separated, the UE may simultaneously perform the measurement onthe legacy network and the CSFB introduction process.

Thereafter, in step 709, the UE sends an RRC connection request messageincluding information (for example, preMeasured flag) indicating thatthe measurements on the legacy network is underway, to the eNB of theLTE system. If the eNB supports the CSFB function, the UE receives fromthe eNB an RRC connection setup message including information (forexample, reportedMearured flag) for instructing to report measurementresults to the eNB, in step 711. In step 713, upon receiving the RRCconnection setup message, the UE sends an RRC connection setup completemessage including the measurement results (for example,preMeasuredResult IE) to the eNB. Upon receiving the measurementresults, the eNB of the LTE system may determine the target cell forCSFB without the separate measurement and reporting process, using thereceived measurement results.

In accordance with an exemplary embodiment in FIG. 7, if a voice calloccurs, the UE may start in advance the measurements on the legacynetwork by means of the separate legacy receiver, and may report themeasurement results on the legacy network to the eNB and the MME throughan RRC connection procedure, thus making it possible to significantlyreduce the time delay compared to the method of executing measurementson the legacy network after receiving a measurement command for thelegacy network from the eNB that has performed the RRC connectionprocedure in the CSFB service.

FIG. 8 is a flow diagram illustrating a communication method in awireless communication system supporting CSFB according to anotherexemplary embodiment of the present invention.

Referring to FIG. 8, in step 801, a voice call occurs as a userinitiates an outgoing call, in the case of an MO call, and a voice calloccurs as a UE receives a paging or CS paging notification message fromthe LTE system, in the case of an MT call. As the voice call occurs, aCSFB process is initiated. The user initiates an outgoing call byexecuting a dial or phonebook function for the outgoing call. If thevoice call occurs, a UE 810 starts measurements on the 2G/3G cell statusin a 2G/3G RAN 830, which is a legacy network, using its legacy receiver507, in step 803.

In step 805, the UE 810 performs a random access to the LTE networkusing its LTE/legacy transceiver 503. Therefore, the UE 810 maysimultaneously perform the measurements on the legacy network and theCSFB introduction process.

In step 807, the UE 810 sends an RRC connection request messageincluding information (for example, preMeasured=yes) indicating that themeasurements on the legacy network is underway by step 803, to an eNB820 of the LTE system. If the eNB 820 supports the CSFB function, the UE810 receives an RRC connection setup message including information (forexample, reportedMearured=yes) for instructing to report the measurementresults, from the eNB 820 in step 809. The preMeasured and thereportedMearured are flag information, and each of them indicates ‘yes’if its value is, for example, ‘1’, and ‘no’ if its value is, forexample, ‘0’. In step 811, the UE 810 sends to the eNB 820 an RRCconnection setup complete message including the measurement results (forexample, preMeasuredResult) obtained in step 803. In step 813, the eNB820 sends an extended service request including the measurement results(for example, preMeasuredResult) on the legacy network to an MME 840 torequest a CSFB process.

In step 815, upon receiving the extended service request, the MME 840signals (or sends) a command to fall back the UE 810 to the 2G/3G legacysystem, to the eNB 820 by using, for example, an S1 interface. In step817, the eNB 820 selects a 2G/3G cell (or a list of cells) to be used bythe UE 810, based on the measurement results (for example,preMeasuredResult) on the legacy network. In step 819, the eNB 820 sendsto the UE 810 a 2G/3G switching command (for example, PS HO command,cell change order, RRC connection release, or the like) to switch to theselected 2G/3G cell. Thereafter, in step 821, the known remainingprocedure for providing the CSFB service to the UE 810 is performed.

FIG. 9 is a flow diagram illustrating a communication method in awireless communication system supporting CSFB according to furtheranother exemplary embodiment of the present invention. Although theembodiment in FIG. 9 is the same as the embodiment in FIG. 8 in startingthe measurements on the legacy network in advance, the embodiment inFIG. 9 separately reports the measurement results on the legacy networkusing the extended service request. Contrary to the embodiment in FIG.8, the embodiment in FIG. 9 may be performed even in a UE in a connectedmode.

Referring to FIG. 9, steps 901 and 903, in which a voice call occurs andthe measurements on the 2G/3G cell status is started in advance, are thesame as steps 801 and 803 in FIG. 8, in terms of the operation. In step905, a UE 910 performs the existing operation for Random Access (RA)/RCCconnection setup. In step 907, the UE 910 sends an extended servicerequest including information (for example, preMeasured=yes) indicatingthat the measurement on the legacy network is underway, to an eNB 920 ofthe LTE system, and the eNB 920 forwards the extended service request toan MME 940.

In step 909, the MME 940 may check from the extended service request theinformation (for example, preMeasured=yes) indicating that the UE 910 isperforming the measurement on the legacy network, or may determine thatthe UE 910 starts the measurement on the legacy network in advance, byreceiving from the UE as in FIG. 10 a Non-Access Stratum (NAS) messageincluding information (for example, Dual RX Configuration=yes)indicating that the UE 910 may perform the measurements in advance, asit has two receivers like, for example, the structure in FIG. 5.

FIG. 10 is a flow diagram illustrating a procedure for sending a NASmessage including Dual RX configuration information according to furtheranother exemplary embodiment of the present invention. The procedure inFIG. 10 may be performed in a UE that includes two receivers (anLTE/legacy transceiver and a legacy receiver) like in the UE structurein FIG. 5.

Referring to FIG. 10, a UE 1010 sends a NAS message including the DualRX configuration information (for example, Dual RX Configuration=yes) toan MME 1040 in step 1001. An Attach message or a Tracking Area Update(TAU) request message used in the LTE system may be used as the NASmessage. In step 1003, upon receiving the NAS message, the MME 1040stores the Dual RX configuration information (that is, capabilityinformation), and may recognize that the UE 1010 has two receivers andstarts in advance the measurements on the legacy network in the CSFBservice. In step 1005, the MME 1040 sends an Attach Approve/Rejectmessage or a UAU Approve/Reject message to the UE 1010 in response tothe Attach message or TAU request message.

Turning back to FIG. 9, having recognized that the UE 910 starts themeasurement on the legacy network in advance, the MME 940 sends a UEContext Modification Request message including information (for example,preMeasured=yes) indicating that the measurement on the legacy networkis underway, to the eNB 920 to start the CSFB service, in step 911.

In step 913, upon receiving the UE Context Modification Request message,the eNB 920 sends, to the UE 910, a UE Information Request messageincluding a command (for example, csfb-ReportReq=yes) to report themeasurement results on the legacy network for the CSFB service insteadof setting for the measurement process.

In step 915, the UE 910 sends an RRC message (for example,UEInformationResponse or MeasurementReport) including the measurementresults (measuredResult) on the legacy network, to the eNB 920 to reportits measurement results. Thereafter, in step 917, the eNB 920 selects a2G/3G cell candidate for the CSFB service based on the measurementresults on the legacy network. Steps 919 and 921, in which the eNB 920sends a 2G/3G switching command and the known remaining procedure forproviding the CSFB service is performed, are the same as steps 819 and821 in FIG. 8, in terms of the operation.

Next, a description will be made of a method for determining which ofthe nearby legacy cells (2G GERAN cell and 3G UTRAN cell) a UE willmeasure first, when the UE initiates measurements on the legacy networkfor the CSFB service in an exemplary embodiment of the presentinvention. If the time required for the measurements on the legacynetwork is long in the CSFB process, the Quality of Experience (QoE) isvery low. Therefore, if there are multiple nearby legacy cells, the UEmay first measure only the cells that can be used for CSFB. The existingCSFB service has no problem, since an eNB selects a target measurementcell to be appropriate for CSFB after it receives a CSFB request andCSFB is started by the MME, and then informs a UE of the selected cell.However, in the case where a UE performs a measurement process on alegacy cell in advance like in the exemplary embodiment of the presentinvention, since the UE does not receive separate measurement settingsfrom an eNB, the eNB may provide in advance information about the cellsthat can be used for the CSFB service, as system information like inFIG. 11.

FIG. 11 is a flow diagram illustrating a procedure for providinginformation about cells for a CSFB service as system information by aneNB according to an exemplary embodiment of the present invention.

Referring to FIG. 11, an eNB 1140 periodically broadcasts a SystemInformation Block (SIB) including measurement information for cellselection in the CSFB service, in steps 1101 and 1103. A SIB6 includesinformation about a 3G (UTRAN) cell, a SIB7 includes information about a2G (GERAN) cell, and the SIB6 and SIB7 may include measurementinformation for the multiple cells that a UE 1110 can measure for cellselection in the CSFB service. The measurement information may includeat least one of information ‘Csfb-preferred’ indicating whether eachcell is a preferred cell during CSFB operation, and information‘csfbPriority’ indicating priority.

For example, the SIB6 transmitted in step 1101 may include at least oneof ‘Csfb-preferred’ and ‘csfbpriority’ in CarrierFreqUTRA-FDD/TDD IEknown in the 3GPP standard. The SIB7 transmitted in step 1103 mayinclude at least one of the Csfb-preferred and csfbpriority incommonInfo IE of CarrierFreqsInfoGERAN IE in the 3GPP standard.

In step 1105, upon receiving the SIB6 or SIB7 transmitted in step 1101or 1103, the UE 1110 may first measure the cell (or frequency), forwhich, for example, the flag of csfb-preferred is set as ‘yes’, or mayfirst measure the cell (or frequency) that has relatively high prioritydepending on the csfbpriority, when starting the measurements for CSFB.

Although not illustrated, the eNB and MME that provide the CSFB serviceaccording to exemplary embodiments of the present invention, which havebeen described with reference to FIGS. 2 to 11, may be implemented toinclude a controller for controlling the operations in FIGS. 2 to 11,and a communication interface for transmitting and receiving messagesincluding various information described with reference to FIGS. 2 to 11.

As is apparent from the foregoing description, according to exemplaryembodiments of the present invention, if a voice call occurs in a UEthat is camping on the LTE system supporting CSFB, the UE may performthe measurements for selecting a 2G/3G cell while performing signalingrequired for the CSFB process in the LTE system, thereby making itpossible to reduce the delay time required until it starts the voicecall, thus contributing to a reduction in power consumption of the UE.

In addition, according to exemplary embodiments of the presentinvention, if a ping-pong phenomenon occurs in which a UE repeats cellselection/reselection between the LTE system and the legacy system inthe LTE system supporting CSFB, the UE may be prevented from failing toreceive a paging message that is sent from the LTE system due to the useof CSFB.

While the invention has been shown and described with reference tocertain exemplary embodiments thereof, it will be understood by thoseskilled in the art that various changes in form and details may be madetherein without departing from the spirit and scope of the invention asdefined by the appended claims and their equivalents.

1. A User Equipment (UE) in a wireless communication system supporting aCircuit Switched FallBack (CSFB) service, the UE comprising: atransceiver configured to transmit and receive a signal between a firstcommunication system or second communication system that uses adifferent frequency; a receiver configured to receive a signal from thesecond communication system; and a controller configured to control anoperation of receiving signaling information for the CSFB service fromthe first communication system through the transceiver, andsimultaneously measuring channel status of the second communicationsystem using the signal received from the receiver.
 2. The UE of claim1, wherein the controller is configured to control an operation of thereceiver to measure the channel status of the second communicationsystem before performing a Radio Resource Control (RRC) connection setupprocedure to the first communication system.
 3. The UE of claim 1,wherein the controller is configured to control an operation ofperforming an RRC connection setup procedure to the first communicationsystem, and reporting measurement results on the channel status of thesecond communication system to an evolved Node B (eNB) of the firstcommunication system.
 4. The UE of claim 3, wherein the measurementresults are reported using an RRC message that is sent to the eNB. 5.The UE of claim 1, wherein the controller is configured to transmitinformation indicating that the measurement of the channel status of thesecond communication system is underway to an eNB of the firstcommunication system, wherein the information is delivered to a MobilityManagement Entity (MME) through the eNB.
 6. The UE of claim 1, whereinthe controller is configured to control an operation of receiving a celllist for the CSFB service which is selected based on measurement resultson the channel status from an eNB of the first communication system. 7.The UE of claim 1, wherein if the UE operates in a connected mode, thecontroller is configured to control an operation of reportingmeasurement results to an eNB upon receiving a command to report themeasurement results on the channel status for the CSFB service from theeNB of the first communication system.
 8. The UE of claim 1, wherein ifthe UE camps on a cell of the second communication system, thecontroller is configured to control an operation of receiving a pagingmessage that is sent for the CSFB service from an eNB of the firstcommunication system through the transceiver.
 9. The UE of claim 1,wherein upon recognizing that a dial program or a phonebook program foran outgoing call of the UE is executed, the controller is configured tostart measurements on the channel status of the second communicationsystem by controlling the receiver.
 10. The UE of claim 1, wherein thecontroller is configured to control an operation of sending, to an MME,a Non-Access Stratum (NAS) message including Dual Reception (RX)configuration information indicating that the UE has two receivers forthe CSFB service.
 11. The UE of claim 1, wherein the controller isconfigured to receive information about cells that can be used for theCSFB service using system information broadcasted by the firstcommunication system, wherein the information about the cells includesat least one of flag information indicating a cell that is firstmeasured for the CSFB service and priority information.
 12. A method forperforming communication by a User Equipment (UE) that uses a CircuitSwitched FallBack (CSFB) service if a voice call of a secondcommunication system occurs in a first communication system providing adata service, comprising: if the voice call occurs, startingmeasurements on channel status of the second communication system usinga receiver for receiving a signal from the second communication system;and sending and receiving messages for the CSFB service with an evolvedNode B (eNB) of the first communication system using a transceiver forcommunication with the first communication system while performing themeasurements on channel status.
 13. The method of claim 12, wherein themeasurement on channel status of the second communication system isstarted before the UE performs a Radio Resource Control (RRC) connectionsetup procedure to the first communication system.
 14. The method ofclaim 12, wherein sending and receiving the messages further comprisesperforming an RRC connection setup procedure to the first communicationsystem, and reporting measurement results on the channel status of thesecond communication system to the eNB of the first communicationsystem.
 15. The method of claim 14, wherein the measurement results arereported using an RRC message that is sent to the eNB.
 16. The method ofclaim 12, wherein sending and receiving the messages further comprisestransmitting information indicating that the measurement on the channelstatus of the second communication system is underway to the eNB of thefirst communication system, wherein the information is delivered to aMobility Management Entity (MME) through the eNB.
 17. The method ofclaim 12, wherein sending and receiving the messages further comprisesreceiving a cell list for the CSFB service which is selected based onmeasurement results on the channel status from the eNB of the firstcommunication system.
 18. The method of claim 12, wherein sending andreceiving the messages further comprises reporting, if the UE operatesin a connected mode, measurement results to the eNB upon receiving acommand to report the measurement results on the channel status for theCSFB service from the eNB of the first communication system.
 19. Themethod of claim 12, further comprising, if the UE camps on a cell of thesecond communication system, receiving a paging message that is sent forthe CSFB service from the eNB of the first communication system throughthe transceiver.
 20. The method of claim 12, wherein starting themeasurements on the channel status further comprises starting themeasurements on the channel status upon recognizing that a dial programor a phonebook program for an outgoing call is executed.
 21. The methodof claim 12, wherein sending and receiving the messages furthercomprises sending, to an MME, a Non-Access Stratum (NAS) messageincluding Dual Reception (RX) configuration information indicating thatthe UE has two receivers for the CSFB service.
 22. The method of claim12, further comprising receiving information about cells that can beused for the CSFB service using system information broadcasted by thefirst communication system, wherein the information about the cellsincludes at least one of flag information indicating a cell that isfirst measured for the CSFB service, and priority information.